Vertical memory device and method for fabricating the same

ABSTRACT

A vertical memory device includes a substrate, a plurality of gate electrodes vertically stacked over the substrate in a cell array region, and a plurality of multi-layered pad portions formed over the substrate in a contact region. Each multi-layered pad portion of the plurality of multi-layered pad portions extends from an end of a gate electrode of the plurality of gate electrodes. Each multi-layered pad portion of the plurality of multi-layered pad portions includes a lower pad, an upper pad spaced vertically apart from the lower pad, a buffer pad formed between the lower pad and the upper pad, and a pad interconnection portion interconnecting the lower pad and the upper pad.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 16/458,785, filed on Jul. 1, 2019, and claimspriority to Korean Patent Application No. 10-2018-0114074, filed on Sep.21, 2018, which is incorporated herein by reference in its entirety.

BACKGROUND 1. Field

Embodiments of the present disclosure relate to a memory device, andmore particularly, to a vertical memory device and a method forfabricating the vertical memory device.

2. Description of the Related Art

Recently, vertical memory devices in which memory cells are stacked overa substrate are being developed. The vertical memory devices includecontact plugs having different heights to electrically connect thememory cells to each other.

SUMMARY

In accordance with an embodiment of the present teachings, a verticalmemory device includes a substrate, a plurality of gate electrodesvertically stacked over the substrate in a cell array region, and aplurality of multi-layered pad portions formed over the substrate in acontact region. Each multi-layered pad portion of the plurality ofmulti-layered pad portions extends from an end of a gate electrode ofthe plurality of gate electrodes. Each multi-layered pad portion of theplurality of multi-layered pad portions includes a lower pad, an upperpad spaced vertically apart from the lower pad, a buffer pad formedbetween the lower pad and the upper pad, and a pad interconnectionportion interconnecting the lower pad and the upper pad.

Also in accordance with an embodiment of the present teachings, a methodfor fabricating a vertical memory device includes forming a plurality ofdielectric layers and a plurality of sacrificial layers interleaved overa substrate in a cell array region and a contact region and forming astepwise structure where the sacrificial layers are exposed bypatterning the dielectric layers and the sacrificial layers in thecontact region. The method also includes forming a plurality ofmulti-layer sacrificial structures including a sacrificial material overthe exposed sacrificial layers of the stepwise structure and forming acapping layer over the sacrificial structures, wherein the capping layerincludes a plurality of air gaps respectively interconnecting themultiple layers of multi-layer sacrificial structures of the pluralityof multi-layer sacrificial structures. The method further includesreplacing the sacrificial layers in the cell array region with aplurality of gate electrodes and replacing the sacrificial layers, theair gaps, and the multi-layer sacrificial structures in the contactregion with a plurality of multi-layered pad portions. The methodadditionally includes forming a plurality of contact plugs, whereincontact plugs of the plurality of contact plugs are respectively coupledto multi-layered pad portions of the multi-layered pad portions.

For an embodiment, the multi-layered pad portions are thicker than thegate electrodes. Replacing the sacrificial layers, the air gaps, and themulti-layer sacrificial structures in the contact region with aplurality of multi-layered pad portions may include forming lower padsand upper pads which are spaced vertically apart from the lower padsthrough replacement of the sacrificial layers and the sacrificialmaterial; and forming pad interconnection portions by filling the airgaps to interconnect the lower pads and the upper pads. Replacing thesacrificial layers in the cell array region with a plurality of gateelectrodes and replacing the sacrificial layers, the air gaps, and themulti-layer sacrificial structures in the contact region with aplurality of multi-layered pad portions may include forming a slit byetching the capping layer, the plurality of sacrificial layers, and theplurality of dielectric layers; forming a plurality of horizontalrecesses by removing the plurality of sacrificial layers in the cellarray region through the slit; forming a plurality of stack-typerecesses with the air gaps embedded therein by removing the plurality ofsacrificial layers and the sacrificial material in the contact regionthrough the plurality of horizontal recesses; and filling the pluralityof horizontal recesses and the plurality of stack-type recesses with aconductive material to form the plurality of gate electrodes and theplurality of multi-layered pad portions. Forming the plurality ofmulti-layer sacrificial structures including a sacrificial material overthe exposed sacrificial layers of the stepwise structure may includedepositing a sacrificial pad, then depositing a buffer pad, and thendepositing an additional sacrificial pad over a sacrificial layer of theplurality of sacrificial layers in the contact region. The sacrificiallayer, the sacrificial pad, the buffer pad, and the additionalsacrificial pad are formed of the same material. The buffer pad and theplurality of dielectric layers may include silicon oxide, and whereinthe plurality of sacrificial layers, the sacrificial pad, the bufferpad, and the additional sacrificial pad may include silicon nitride.Forming a plurality of multi-layer sacrificial structures including asacrificial material over the exposed sacrificial layers of the stepwisestructure may include forming a sacrificial pad layer over a sacrificiallayer; etching the sacrificial pad layer to form a sacrificial pad;sequentially depositing a buffer layer and an additional sacrificial padlayer over the sacrificial pad; forming a sacrificial buffer pad overthe additional sacrificial pad layer; etching the additional sacrificialpad layer with the sacrificial buffer pad used as an etch mask to formthe additional sacrificial pad; and etching the buffer layer to form thebuffer pad, wherein the additional sacrificial pad and the sacrificialpad form a multi-layer sacrificial structure of the plurality of themulti-layer sacrificial structures. A method may further includeforming, after forming the capping layer, a vertical channel structurethat penetrates through the capping layer, the plurality of dielectriclayers, and the plurality of sacrificial layers in the cell arrayregion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view illustrating a semiconductor devicein accordance with an embodiment of the present teachings.

FIG. 2 shows a cross-sectional view illustrating a modified example of apad portion.

FIGS. 3A to 3C show cross-sectional views illustrating other modifiedexamples of the pad portion.

FIGS. 4A and 4B illustrate a vertical memory device.

FIGS. 5A to 5P show cross-sectional views describing an example of amethod for fabricating a vertical memory device.

FIGS. 6A and 6B show cross-sectional views illustrating a pad portionaccording to comparative examples.

DETAILED DESCRIPTION

Embodiments of the present teachings are described below in detail withreference to the accompanying drawings. The present teachings may,however, be embodied in different forms and should not be construed aslimited to the embodiments set forth herein. The presented embodimentsare provided so that this disclosure will enable those skilled in theart to practice the present teachings without undue experimentation.Throughout the disclosure, like reference numerals refer to like partsillustrated in the various figures.

The figures are not necessarily drawn to scale, and in some instances,relative proportions may be exaggerated in order to clearly illustratefeatures of an embodiment. When a first layer is referred to as being“on” a second layer or “on” a substrate, it not only refers to a casewhere the first layer is formed directly on the second layer or thesubstrate but also a case where a third layer exists between the firstlayer and the second layer or the substrate.

Various embodiments of the present disclosure are directed to a verticalmemory device with improved reliability and a method for fabricating thevertical memory device. For various embodiments, a vertical memorydevice has memory strings which are vertically oriented, such that atleast one memory cell is located over another memory cell. This type ofarray allows for vertical scaling to provide a higher density of memorycells per unit area of silicon or other semiconductor material.

FIG. 1 shows a cross-sectional view illustrating a semiconductor device,in accordance with an embodiment.

Referring to FIG. 1, a semiconductor device 100 may include a firstconductive structure 103 positioned over a substrate 101. A dielectricmaterial 102 may be positioned between the substrate 101 and the firstconductive structure 103. The semiconductor device 100 may furtherinclude a second conductive structure 104 coupled to the firstconductive structure 103.

The first conductive structure 103 may extend horizontally with thesubstrate 101. The first conductive structure 103 may include a baseportion 103B and a pad portion 103P. The pad portion 103P may extendfrom one end of the base portion 103B. The pad portion 103P may bereferred to as ‘a contact area’. The pad portion 103P may be thickerthan the base 103B. The base portion 103B may be a single layer, and thepad portion 103P may have multiple layers. The base portion 103B and thepad portion 103P may include the same material. The first conductivestructure 103 may include a semiconductor material, a metal, a metalcompound, or a combination thereof. The first conductive structure 103may include polysilicon, tungsten, a titanium nitride, or a combinationthereof. The second conductive structure 104 may be formed perpendicularto the surface of the substrate 101. The second conductive structure 104may have the shape of a pillar. The second conductive structure 104 mayinclude a semiconductor material, a metal, a metal compound, or acombination thereof. The second conductive structure 104 may includepolysilicon, tungsten, a titanium nitride, or a combination thereof. Thefirst conductive structure 103 and the second conductive structure 104may be formed of the same material or may be formed of differentmaterials. The second conductive structure 104 may be in contact withthe pad portion 103P of the first conductive structure 103. The firstconductive structure 103 and the second conductive structure 104 mayelectrically contact each other.

The pad portion 103P of the first conductive structure 103 may include alower pad 103L, an upper pad 103U, and a pad interconnection portion1031. The lower pad 103L and the upper pad 103U may be coupled to eachother by the pad interconnection portion 1031. The lower pad 103L andthe upper pad 103U may be coupled to one end of the base portion 103B atthe same time. For example, one end of the base portion 103B may becoupled to the lower pad 103L and the upper pad 103U by the padinterconnection portion 1031. The upper pad 103U may be in directcontact with the second conductive structure 104. A recess 103R may beformed on an upper surface of the upper pad 103U. The bottom surface ofthe second conductive structure 104 may be positioned in the recess103R.

The lower pad 103L may be positioned at the same level as the baseportion 103B, and the upper pad 103U may be positioned at a higher levelthan the base portion 103B. The base portion 103B may have a firstthickness D1, the lower pad 103L may have a second thickness D2, and theupper pad 103U may have a third thickness D3. The lower pad 103L and theupper pad 103U may have the same thickness (D2=D3). The lower pad 103Lmay be thicker than the base portion 103B (D2>D1). The total thicknessof the lower pad 103L and the upper pad 103U may be thicker than thebase 103B. The lengths of the lower pad 103L and the upper pad 103U maybe the same. The base portion 103B may be longer than the pad portion103P.

A buffer pad 105 may be positioned between the lower pad 103L and theupper pad 103U. The buffer pad 105 may include a dielectric material.The buffer pad 105 may be thinner than the lower pad 103L and the upperpad 103U. The buffer pad 105 may be thinner than the base portion 103B.

Referring to FIG. 1, the semiconductor device 100 may be part of amemory device. The semiconductor device 100 may be part of a verticalmemory device. The semiconductor device 100 may be part of a verticalNAND flash memory device. For example, the first conductive structure103 may be a gate electrode, and the second conductive structure 104 maybe a contact plug. According to another embodiment, the first conductivestructure 103 may be a bit line or a metal line, and the secondconductive structure 104 may be a contact plug.

As described above, because the pad portion 103P includes the lower pad103L and the upper pad 103U, even if the second conductive structure 104penetrates through the upper pad 103U, the lower pad 103L may bemaintained stably. Also, because the upper pad 103U is relativelythicker than the base portion 103B, the punch phenomenon of the secondconductive structure 104 may be suppressed. Because the buffer pad 105is positioned between the lower pad 103L and the upper pad 103U, thelower pad 103L and the upper pad 103U may be protected from collapsingwhen the second conductive structure 104 is formed. In some embodiments,the buffer pad 105 may be referred to as a support pad.

The pad portion 103P of FIG. 1 is a multi-layered pad structure and mayhave the form of a double pad including the lower pad 103L and the upperpad 103U.

FIG. 2 shows a cross-sectional view illustrating a modified example ofthe pad portion, indicated at 103P′.

Referring to FIG. 2, the pad portion 103P′ may include the lower pad103L and an upper pad 103U′. The pad portion 103P′ may further includethe pad interconnection portion 1031 coupling the lower pad 103L and theupper pad 103U′ to each other. The pad portion 103P′ may further includethe buffer pad 105 between the lower pad 103L and the upper pad 103U′.

In the pad portion 103P′ of FIG. 2, the upper pad 103U′ may be thinnerthan the lower pad 103L (D3′<D2). The upper pad 103U′ may be thickerthan the base portion 103B (D3′>D1). The total thickness of the lowerpad 103L and the upper pad 103U′ may be thicker than the base portion103B.

The pad portion 103P′ of FIG. 2 may have the form of a double padincluding the lower pad 103L and the upper pad 103U′.

FIGS. 3A to 3C show cross-sectional views illustrating other modifiedexamples of the pad portion consistent with the present teachings.

Referring to FIGS. 3A to 3C, pad portions 103P11, 103P12, and 103P13 mayhave a multi-layered pad structure, e.g., the pad portions 103P11,103P12, and 103P13 may have a triple pad shape. The pad portions 103P11,103P12, and 103P13 may be in contact with the second conductivestructure 104.

Referring to FIG. 3A, the pad portion 103P11 may include a lower pad103L, an upper pad 103U, and a middle pad 103M positioned between thelower pad 103L and the upper pad 103U. The lower pad 103L, the upper pad103U, and the middle pad 103M may be coupled to each other by a padinterconnection portion 103. The middle pad 103M may have a thicknessD4. The middle pad 103M may have the same thickness as the lower pad103L and the upper pad 103U (D4=D2=D3). The middle pad 103M may bethicker than the base portion 103B (D4>D1). The middle pad 103M may bemade of the same material as the lower pad 103L and the upper pad 103U.A first buffer pad 105A may be positioned between the lower pad 103L andthe middle pad 103M. A second buffer pad 105B may be positioned betweenthe upper pad 103U and the middle pad 103M. The first and second bufferpads 105A and 105B may be made of a dielectric material. The first andsecond buffer pads 105A and 105B may be thinner than the middle pad103M, the lower pad 103L, and the upper pad 103U.

Referring to FIG. 3B, the pad portion 103P12 may include a lower pad103L′, an upper pad 103U′, and a middle pad 103M′ positioned between thelower pad 103L′ and the upper pad 103U′. The lower pad 103L′, the upperpad 103U′, and the middle pad 103M′ may be coupled to each other by apad interconnection portion 1031. The middle pad 103M′ may havethickness D4′. The middle pad 103M′ may have the same thickness as thelower pad 103L′ and the upper pad 103U′(D4′=D2′=D3′. The lower pad 103L′and the upper pad 103U′ may be as thick as the base portion 103B(D1=D2′=D3′=D4′). The middle pad 103M′ may be made of the same materialas the lower pad 103L′ and the upper pad 103U′. A first buffer pad 105Amay be positioned between the lower pad 103L′ and the middle pad 103M′.A second buffer pad 105B may be positioned between the upper pad 103U′and the middle pad 103M′. The first and second buffer pads 105A and 105Bmay be made of a dielectric material. The first and second buffer pads105A and 105B may be thinner than the middle pad 103M′, the lower pad103L′, and the upper pad 103U′.

Referring to FIG. 3C, the pad portion 103P13 may include a lower pad103L, an upper pad 103U′, and a middle pad 103M′ positioned between thelower pad 103L and the upper pad 103U′. The lower pad 103L, the upperpad 103U′, and the middle pad 103M′ may be coupled to each other by apad interconnection portion 1031. The middle pad 103M′ may have athickness D4′. The middle pad 103M′ may be thinner than the lower pad103L (D4′<D2). The middle pad 103M′ and the upper pad 103U′ may have thesame thickness (D3′=D4′). The middle pad 103M′ and the upper pad 103U′may have the same thickness as the base portion 103B (D1=D3′=D4′). Thelower pad 103L may be thicker than the base portion 103B (D2>D1). Themiddle pad 103M′ may be made of the same material as the lower pad 103Land the upper pad 103U′. A first buffer pad 105A may be positionedbetween the lower pad 103L and the middle pad 103M′. A second buffer pad105B may be positioned between the upper pad 103U′ and the middle pad103M′. The first and second buffer pads 105A and 105B may be made of adielectric material. The first and second buffer pads 105A and 105B maybe thinner than the middle pad 103M′, the lower pad 103L, and the upperpad 103U′.

Referring to FIGS. 3A to 3C, each of the pad portions 103P11, 103P12,and 103P13 is formed in a triple pad shape so that a punch margin may bemaximized when the second conductive structure 104 is formed.

The pad portions 103P, 103P′, 103P11, 103P12, and 103P13, according tothe above-described embodiments, may be part of a memory device. The padportions 103P, 103P′, 103P11, 103P12, and 103P13 may be part of a gateelectrode. The gate electrode may be part of a word line, a control gateelectrode, or a select gate electrode. The word line, the control gateelectrode, and the select gate electrode may be part of a non-volatilememory device. The word line, the control gate electrode, and the selectgate electrode may be part of a vertical NAND flash memory device.

FIGS. 4A and 4B illustrate a vertical memory device 200. FIG. 4A shows aplan view of the vertical memory device 200, and FIG. 4B shows across-sectional view of the vertical memory device 200 taken along aline A-A′ of FIG. 4A.

Referring to FIGS. 4A and 4B, the vertical memory device 200 may includea plurality of cell strings STR. Each of the cell strings STR mayinclude a cell array region MC and a contact region CR. A plurality ofgate electrodes 221, 222, and 223 and a plurality of dielectric layers231, 232, and 233 may be alternately stacked over an upper surface ofthe substrate 201. The plurality of gate electrodes 221 to 223 and theplurality of dielectric layers 231 to 233 may be formed over both thecell array region MC and the contact region CR. A plurality of verticalchannel structures 250 penetrating through the gate electrodes 221 to223 and the dielectric layers 231 to 233 may be formed. The plurality ofvertical channel structures 250 may be formed in the cell array regionMC. Neighboring cell strings STR may be isolated by slits 202. From theperspective of a top view, the vertical channel structures 250 may beregularly arrayed. For convenience of explanation, three verticalchannel structures 250 are shown in one cell string STR. In otherembodiments, different numbers of the vertical channel structures 250may be arranged in one cell string STR. For convenience of explanation,three dielectric layers 231 to 233 and three gate electrodes 221 to 223are shown alternately stacked in FIG. 4B. In other embodiments,different numbers of the dielectric layers 231 to 233 and the gateelectrodes 221 to 223 may be alternately stacked. Each of the verticalchannel structures 250 may include a charge storage layer 251 which ispositioned adjacent to the gate electrodes 221 to 223, a tunneldielectric layer 252 which is in contact with the charge storage layer251, and a channel layer 253 which is in contact with the tunneldielectric layer 252. The inner space of the channel layer 253 may befilled with a core dielectric layer 254. A conductive pad 255 may beformed over the core dielectric layer 254. The tunnel dielectric layer252 may surround the outer wall of the channel layer 253. The chargestorage layer 251 may surround the outer wall of the tunnel dielectriclayer 252.

The gate electrodes 221 to 223 may include a conductive material. Thegate electrodes 221 to 223 may include titanium nitride, tungsten, or astack thereof. Each of the gate electrodes 221 to 223 may include baseportions 221B, 222B, and 223B and pad portions 241, 242, and 243. Thebase portions 221B, 222B, and 223B may be positioned over the cell arrayregion MC. A portion of the base portions 221B, 222B, and 223B mayextend over the contact region CR. The pad portions 241, 242, and 243may be positioned over the contact region CR. The pad portions 241, 242,and 243 may extend horizontally from the ends of the base portions 221B,222B, and 223B. The pad portions 241, 242, and 243 may be thicker thanthe base portions 221B, 222B, and 223B. The base portions 221B, 222B,and 223B and the pad portions 241, 242, and 243 may include the samematerial. Each of the base portions 221B, 222B, and 223B may be a singlelayer, and each of the pad portions 241, 242, and 243 may be multiplelayers. Accordingly, the base portions 221B, 222B, and 223B among thegate electrodes 221 to 223 may be referred to as a single-layered gateelectrodes, and the pad portions 241, 242, and 243 may be referred to asa multi-layered pad portions.

The gate electrodes 221 to 223 may be referred to as a word line. Thedielectric layers 231 to 233 will be referred to as a first dielectriclayer 231, a second dielectric layer 232, and a third dielectric layer233 in a direction perpendicular to the upper surface of the substrate201. The gate electrodes 221 to 223 are referred to as a first gateelectrode 221, a second gate electrode 222, and a third gate electrode223. The pad portions 241, 242, and 243 may be arranged in a stepwisestructure. The pad portions 241, 242, and 243 are referred to as a firstpad portion 241, a second pad portion 242, and a third pad portion 243.The upper surface of the first pad portion 241 may be positioned at alower level than the upper surface of the second pad portion 242, andthe upper surface of the second pad portion 242 may be positioned at alower level than the upper surface of the third pad portion 243. Thefirst pad portion 241 may be coupled to one end of the first gateelectrode 221, and the second pad portion 242 may be coupled to one endof the second gate electrode 222. The third pad portion 243 may becoupled to one end of the third gate electrode 223.

A plurality of contact plugs 271, 272, and 273 may be coupled to the padportions 241, 242, and 243, respectively. For example, the contact plugs271, 272, and 273 may include a first contact plug 271, a second contactplug 272, and a third contact plug 273. The first contact plug 271 maybe coupled to the first pad portion 241, the second contact plug 272 maybe coupled to the second pad portion 242, and the third contact plug 273may be coupled to the third pad portion 243.

The contact plugs 271, 272, and 273 may penetrate through an inter-layerdielectric layer 260. The inter-layer dielectric layer 260 may be formedover the entire profile of the substrate 201 while covering the padportions 241, 242, and 243. The inter-layer dielectric layer 260 mayinclude a capping layer 261 and a planarization layer 262. Theplanarization layer 262 may be thicker than the capping layer 261. Thevertical channel structure 250 and the contact plugs 271, 272, and 273may all penetrate through the inter-layer dielectric layer 260.

Referring to FIG. 4B, the pad portions 241, 242, and 243 mayrespectively include the lower pads 241L, 242L, and 243L, the upper pads241U, 242U, and 243U, and the pad interconnection portions 241I, 242I,and 243I. The pad interconnection portions 241I, 242I, and 243I mayinterconnect the lower pads 241L, 242L, and 243L with the upper pads241U, 242U, and 243U, respectively. The pad portions 241, 242, and 243may further include buffer pads 241B, 242B, and 243B. The buffer pads241B, 242B, and 243B may be positioned between the lower pads 241L,242L, and 243L and the upper pads 241U, 242U, and 243U, respectively. Aportion of the buffer pads 241B, 242B, 243B may be in contact with thedielectric layers 231, 232, and 233, respectively. The buffer pads 241B,242B, and 243B and the capping layer 261 may prevent the neighboring padportions 241, 242, and 243 from being bridged. The buffer pads 241B,242B, and 243B may prevent the pad portions 241, 242, and 243 fromcollapsing by providing structural support. The buffer pads 241B, 242B,and 243B may include a dielectric material. The buffer pads 241B, 242B,and 243B may include silicon oxide. The lower pads 241L, 242L, and 243L,the upper pads 241U, 242U, and 243U, and the pad interconnectionportions 241I, 242I, and 243I may include a conductive material. Thelower pads 241L, 242L, and 243L, the upper pads 241U, 242U, and 243U,and the pad interconnection portions 241I, 242I, and 243I may includetitanium nitride, tungsten, or a stack thereof.

The lower pads 241L, 242L, and 243L and the upper pads 241U, 242U, and243U may have the same thickness. The lower pads 241L, 242L, and 243Land the upper pads 241U, 242U, and 243U may be thicker than the baseportions 221B, 222B, and 223B. The buffer pads 241B, 242B, and 243B maybe thinner than the lower pads 241L, 242L, and 243L and the upper pads241U, 242U, and 243U.

According to another embodiment, the pad portions 241, 242, and 243 mayhave a structure including a double pad shape and a triple pad shapeshown in FIGS. 2 and 3A to 3C.

FIGS. 5A to 5P show cross-sectional views used in describing a methodfor fabricating a vertical memory device.

Referring to FIG. 5A, a dielectric layer stack 20 and a sacrificiallayer stack 30 may be formed over a substrate 11. The substrate 11 mayinclude a cell array region MC and a contact region CR. The dielectriclayer stack 20 may include a plurality of dielectric layers 21, 22, and23. The sacrificial layer stack 30 may include a plurality ofsacrificial layers 31, 32, and 33. The dielectric layers 21 to 23 andthe sacrificial layers 31 to 33 may be alternately stacked. One layeramong the dielectric layers 21 to 23 together with one layer among thesacrificial layers 31 to 33 may form a dielectric layer-sacrificiallayer pair. For example, a plurality of dielectric layer-sacrificiallayer pairs P1, P2, and P3 may be sequentially formed over the substrate11. Each of the dielectric layer-sacrificial layer pairs P1 to P3 mayextend parallel to the surface of the substrate 11. The dielectriclayers 21 to 23 and the sacrificial layers 31 to 33 may be alternatelystacked in a direction perpendicular to the surface of the substrate 11.The dielectric layer-sacrificial layer pairs P1 to P3 may be ofdifferent lengths so that the dielectric layer-sacrificial layer pairsP1 to P3 may form a stepwise structure 40. The stepwise structure 40 maybe formed in the contact region CR. A plurality of pad regions 41, 42,and 43 may be defined by the stepwise structure 40. Portions of thesacrificial layers 31 to 33 may be exposed by the stepwise structure 40.For example, as one method for forming the stepwise structure 40, a masklayer (not shown) may be formed after the dielectric layer stack 20 andthe sacrificial layer stack 30 are formed, and the sacrificial layerstack 30 and the dielectric layer stack 20 that are exposed through themask layer may be etched. Subsequently, a process of etching thesacrificial layer stack 30 and the dielectric layer stack 20 exposedthrough the mask layer while trimming the mask layer may be performed aplurality of times. For example, the dielectric layer-sacrificial layerpair P1 may be formed first, and then the dielectric layer-sacrificiallayer pair P3 may be formed later. The sacrificial layers 31 to 33 maybe positioned over the dielectric layers 21 to 23, respectively, in eachof the dielectric layer-sacrificial layer pairs P1 to P3. The padregions 41 to 43 may include a first pad region 41, a second pad region42, and a third pad region 43.

The dielectric layers 21 to 23 and the sacrificial layers 31 to 33 maybe formed of a material having an etch selectivity. The etch selectivitymay be defined as the ratio of the etch rate of the sacrificial layers31 to 33 to the etch rate of the dielectric layers 21 to 23. Thedielectric layers 21 to 23 may be of a material that is not removed whenthe sacrificial layers 31 to 33 are etched. The dielectric layers 21 to23 may be formed of silicon oxide, and the sacrificial layers 31 to 33may be formed of a material having an etch selectivity to the dielectriclayers 21 to 23. For example, the dielectric layers 21 to 23 may besilicon oxide, and the sacrificial layers 31 to 33 may be siliconnitride. According to another embodiment, the sacrificial layers 31 to33 may include a silicon layer or silicon carbide.

Referring to FIG. 5B, a sacrificial pad layer 50 may be formed. Thesacrificial pad layer 50 may be formed over the entire profile of thesubstrate 11, including the stepwise structure 40. The sacrificial padlayer 50 may cover the pad regions 41, 42, and 43. The sacrificial padlayer 50 and the sacrificial layers 31, 32, and 33 may be formed of thesame material. The sacrificial pad layer 50 and the dielectric layers21, 22, and 23 may be formed of different materials. The sacrificial padlayer 50 may be formed of a material having an etch selectivity withrespect to the dielectric layers 21 to 23. The sacrificial pad layer 50may include silicon nitride, and the dielectric layers 21 to 23 mayinclude silicon oxide.

Referring to FIG. 5C, a plurality of sacrificial pads 51P, 52P, and 53Pmay be formed. The sacrificial pads 51P to 53P may be formed in the padregions 41, 42, and 43, respectively. The sacrificial pad layer 50 maybe etched to form the sacrificial pads 51P to 53P. The sacrificial pads51P to 53P might not contact the dielectric layers 21 to 23. Thesacrificial pads 51P to 53P may include a first sacrificial pad 51P, asecond sacrificial pad 52P, and a third sacrificial pad 53P. The firstsacrificial pad 51P may be formed in the first pad region 41, the secondsacrificial pad 52P may be formed in the second pad region 42, and thethird sacrificial pad 53P may be formed in the third pad region 43. Thefirst sacrificial pad 51P, the second sacrificial pad 52P, and the thirdsacrificial pad 53P may have the same thickness. The first sacrificialpad 51P, the second sacrificial pad 52P, and the third sacrificial pad53P may have the same length. The first sacrificial pad 51P and thesecond dielectric layer 22 may be positioned at the same level. A firstgap 51V may be formed between the first sacrificial pad 51P and thesecond dielectric layer 22. The second sacrificial pad 52P and the thirddielectric layer 23 may be positioned at the same level. A second gap52V may be formed between the second sacrificial pad 52P and the thirddielectric layer 23. The first sacrificial pad 51P and the secondsacrificial pad 52P may be spaced apart from each other by the first gap51V. The second sacrificial pad 52P and the third sacrificial pad 53Pmay be spaced apart from each other by the second gap 52V.

Referring to FIG. 5D, a buffer layer 60 may be formed. The buffer layer60 may be formed over the entire profile of the substrate 11, includingthe sacrificial pads 51P to 53P. The buffer layer 60 may cover aplurality of sacrificial pads 51P to 53P. The buffer layer 60 may fillthe first gap 51V and the second gap 52V. The buffer layer 60 and thedielectric layers 21 to 23 may be formed of the same material. Thebuffer layer 60 and the sacrificial layers 31 to 33 may be formed ofdifferent materials. The buffer layer 60 may be formed of a materialhaving an etch selectivity with respect to the sacrificial layers 31 to33 and the sacrificial pads 51P to 53P. The buffer layer 60 may includesilicon oxide. A portion of the buffer layer 60 may cover the side wallsof the sacrificial layers 31 to 33. The first sacrificial pad 51P andthe second sacrificial pad 52P may be spaced apart from each other bythe buffer layer 60. The second sacrificial pad 52P and the thirdsacrificial pad 53P may be spaced apart from each other by the bufferlayer 60.

Referring to FIG. 5E, an additional sacrificial pad layer 50′ may beformed. The additional sacrificial pad layer 50′ may be formed over theentire profile of the substrate 11, including the buffer layer 60. Theadditional sacrificial pad layer 50′ may cover the pad regions 41 to 43.The additional sacrificial pad layer 50′ and the sacrificial layers 31to 33 may be formed of the same material. The additional sacrificial padlayer 50′ and the sacrificial pads 51P to 53P may be formed of the samematerial. The additional sacrificial pad layer 50′ and the dielectriclayer 21 to 23 may be formed of different materials. The additionalsacrificial pad layer 50′ and the buffer layer 60 may be formed ofdifferent materials. The additional sacrificial pad layer 50′ may beformed of a material having an etch selectivity with respect to thedielectric layer 21 to 23 and the buffer layer 60. The additionalsacrificial pad layer 50′ may include silicon nitride, and thedielectric layers 21 to 23 and the buffer layer 60 may include siliconoxide. The additional sacrificial pad layer 50′ may be thicker than thesacrificial pads 51P to 53P. The additional sacrificial pad layer 50′may be formed to a thickness of approximately 400 Å. In variousembodiments, for example, additional sacrificial pad layer 50′ may beformed to a thickness of between 300 Å and 500 Å. The thickness of theadditional sacrificial pad layer 50′ may be the same as the totalthickness of one sacrificial layer among the sacrificial layers 31 to 33and the sacrificial pad layer 50. The additional sacrificial pad layer50′ and the sacrificial pads 51P to 53P may be spaced apart from eachother by the buffer layer 60.

Referring to FIG. 5F, a sacrificial buffer layer 60′ may be formed. Thesacrificial buffer layer 60′ may be formed over the entire profile ofthe substrate 11, including the additional sacrificial pad layer 50′.The sacrificial buffer layer 60′ and the buffer layer 60 may be formedof the same material. The sacrificial buffer layer 60′ and theadditional sacrificial pad layer 50′ may be formed of differentmaterials. The sacrificial buffer layer 60′ may be formed of a materialhaving an etch selectivity with respect to the additional sacrificialpad layer 50′. The sacrificial buffer layer 60′ may include siliconoxide, and the additional sacrificial pad layer 50′ may include siliconnitride. The sacrificial buffer layer 60′ may be thinner than theadditional sacrificial pad layer 50′. The sacrificial buffer layer 60′and the buffer layer 60 may have the same thickness.

Referring to FIG. 5G, sacrificial buffer pads 61P′, 62P′, and 63P′ maybe formed. The sacrificial buffer pads 61P′ to 63P′ may be formed in thepad regions 41 to 43. In order to form the sacrificial buffer pads 61P′to 63P′, the sacrificial buffer layer 60′ may be etched by using a maskpattern (not shown). The sacrificial buffer layer 60′ may be wet-etchedto form the sacrificial buffer pads 61P′ to 63P′. Portions of theadditional sacrificial pad layer 50′ may be exposed as the sacrificialbuffer pads 61P′ to 63P′ are formed.

Referring to FIG. 5H, a plurality of additional sacrificial pads 51P′,52P′, and 53P′ may be formed. The additional sacrificial pad layer 50′may be etched by using the sacrificial buffer pads 61P′ to 63P′ as anetch mask to form the plurality of additional sacrificial pads 51P′ to53P′. The additional sacrificial pad layer 50′ may be wet-etched to formthe additional sacrificial pads 51P′ to 53P′. The plurality ofadditional sacrificial pads 51P′ to 53P′ may be formed in the padregions 41 to 43. After forming the additional sacrificial pads 51P′ to53P′, portions of the buffer layer 60 may be exposed. The additionalsacrificial pads 51P′ to 53P′ may include a first additional sacrificialpad 51P′, a second additional sacrificial pad 52P′, and a thirdadditional sacrificial pad 53P′.

A first pad gap 51V′ may be formed between the first additionalsacrificial pad 51P′ and the second additional sacrificial pad 52P′. Asecond pad gap 52V′ may be formed between the second additionalsacrificial pad 52P′ and the third additional sacrificial pad 53P′. Thefirst additional sacrificial pad 51P′ and the second additionalsacrificial pad 52P′ may be spaced apart from each other by the firstpad gap 51V′. The second additional sacrificial pad 52P′ and the thirdadditional sacrificial pad 53P′ may be spaced apart from each other bythe second pad gap 52V′. The upper surface of the first additionalsacrificial pad 51P′ may be positioned at a level lower than the uppersurface of the second additional sacrificial pad 52P′. The upper surfaceof the second additional sacrificial pad 52P′ may be positioned at alevel lower than the upper surface of the third additional sacrificialpad 53P′.

Referring to FIG. 5, buffer pads 61P, 62P, and 63P may be formed. Inorder to form the buffer pads 61P to 63P, the buffer layer 60 may beetched by using the additional sacrificial pads 51P′ to 53P′ as anetching mask. The buffer layer 60 may be dry-etched to form the bufferpads 61P to 63P. When the buffer layer 60 is etched, the sacrificialbuffer pads 61P′ to 63P′ may be removed. After the buffer pads 61P to63P are formed, a portion of the sacrificial pads 51P, 52P, and 53P maybe exposed. The buffer pads 61P to 63P may include a first buffer pad61P, a second buffer pad 62P, and a third buffer pad 63P. A first padgap 51V″ may be formed between the first buffer pad 61P and the secondbuffer pad 62P, and a second pad gap 52V″ may be formed between thesecond buffer pad 62P and the third buffer pad 63P. The first buffer pad61P and the second buffer pad 62P may be spaced apart from each other bythe first pad gap 51V″. The second buffer pad 62P and the third bufferpad 63P may be spaced apart from each other by the second pad gap 52V″.The upper surface of the first buffer pad 61P may be positioned at alevel lower than the upper surface of the second buffer pad 62P. Theupper surface of the second buffer pad 62P may be positioned at a levellower than the upper surface of the third buffer pad 63P. The first padgap 51V″ and the second pad gap 52V″ may extend down beyond the bottomof the first pad gap 51V′ and the second pad gap 52V′, respectively,through the buffer layer 60.

As described above, by forming the buffer pads 61P to 63P, a pluralityof protruding structures 64P, 65P, and 66P may be formed in the contactregion CR. The protruding structures 64P, 65P, and 66P may be formed asa stepwise structure. The protruding structures 64P, 65P, and 66P may bestacked over the sacrificial layers 31 to 33, respectively, in the orderof the sacrificial pads 51P to 53P, the buffer pads 61P to 63P, and theadditional sacrificial pads 51P′ to 53P′. Each of the protrudingstructures 64P, 65P, and 66P may be referred to as a multi-layeredsacrificial structure. In other words, each of the protruding structures64P, 65P, 66P may include the sacrificial layers 31 to 33, thesacrificial pads 51P to 53P, and the additional sacrificial pads 51P′ to53P′, respectively. Each of the sacrificial layers 31 to 33 formed inthe cell array region MC may have a single-layer structure. For anembodiment, the sacrificial layers 51P to 53P and the additionalsacrificial layers 51P′ to 53P′ are not formed in the cell array regionMC, and only the sacrificial layers 31 to 33 are be formed in the cellarray region MC.

As shown in FIG. 5J, a capping layer 71 may be formed. A planarizationlayer 72 may be formed over the capping layer 71. The capping layer 71may be formed over the entire profile of the substrate 11, including theadditional sacrificial pads 51P′ to 53P′. The capping layer 71 may coverthe additional sacrificial pads 51P′ to 53P′. When the capping layer 71is formed, air gaps V1, V2, and V3 may be formed over the sacrificialpads 51P, 52P, and 53P and adjacent to the additional sacrificial pads51P′, 52P′, and 53P′, respectively. In order to form the air gaps V1 toV3, the capping layer 71 may be formed non-conformally. For example, thecapping layer 71 may have poor step coverage to form the air gaps V1 toV3. The capping layer 71 may be formed using a plasma-based depositionmethod, such as Plasma Enhanced Chemical Vapor Deposition (PECVD) orPlasma Enhanced Atomic Layer Deposition (PEALD). The capping layer 71and the planarization layer 72 may be collectively referred to as aninter-layer dielectric layer. The capping layer 71 may include siliconoxide. For example, the capping layer 71 may include Undoped SilicateGlass (USG).

The air gaps V1 to V3 may include a first air gap V1, a second air gapV2, and a third air gap V2. The first air gap V1 may be positioned onthe side wall of the first buffer pad 61P. The second air gap V2 may bepositioned on the side wall of the second buffer pad 62P. The third airgap V3 may be positioned on the side wall of the third buffer pad 63P.The first air gap V1 may interconnect the first sacrificial pad 51P andthe first additional sacrificial pad 51P′. The second air gap V2 mayinterconnect the second sacrificial pad 52P and the second additionalsacrificial pad 52P′. The third air gap V3 may interconnect the thirdsacrificial pad 53P and the third additional sacrificial pad 53P′.

Referring to FIG. 5K, a vertical hole 12 may be formed. The verticalhole 12 may be formed perpendicular to the surface of the substrate 11.The vertical hole 12 may be formed by etching the dielectric layers 21to 23, the sacrificial layers 31 to 33, the capping layer 71, and theplanarization layer 72. The vertical hole 12 may be formed in the cellarray region MC. Although not illustrated, the vertical hole 12 may beformed in plural, and the plurality of such holes may have an arraystructure from the perspective of a plan view. When the vertical holes12 are formed, the surface of the substrate 11 may be over-etched.

Referring to FIG. 5L, a vertical channel structure 13 may be formed. Thevertical channel structure 13 may fill the vertical holes 12. Thevertical channel structure 13 may correspond to the vertical channelstructure 250 of FIG. 4B. The vertical channel structure 13 may bereferred to as ‘a pillar structure.’

Subsequently, referring to FIGS. 5M and 5N, a process of replacing thesacrificial layers 31 to 33 and the protruding structures 64P, 65P, and66P with gate electrodes 91G, 92G, and 93G and pad portions 91P, 92P,and 93P may be performed.

Referring to FIG. 5M, a plurality of recesses 81, 82, and 83 may beformed. In order to form the recesses 81 to 83, a slit (see 202 in FIG.4A) may be formed first. The sacrificial layers 31 to 33 may be removedthrough the slit, and the recesses 81 to 83 may be formed between thedielectric layers 21 to 23. The sacrificial layers 31 to 33 may beremoved through a wet etch process. For example, when the sacrificiallayers 31 to 33 include silicon nitride, the sacrificial layers 31 to 33may be removed through a wet etching process using a phosphoric acid(H₃PO₄) solution. The recesses 81 to 83 may be parallel to the surfaceof the substrate 11. The recesses 81 to 83 may be referred to ashorizontal recesses. The recesses 81 to 83 may expose the sidewalls ofthe vertical channel structure 13.

The recesses 81 to 83 may include a first recess 81, a second recess 82,and a third recess 83. Each of the first recess 81, the second recess82, and the third recess 83 may have a first stack-type recess 81R, asecond stack-type recess 82R, and a third stack-type recess 83R that arepositioned in the contact region CR. The sacrificial layers 31 to 33,the sacrificial pads 51P to 53P, and the additional sacrificial pads51P′ to 53P′ may be removed in order to form the first stack-type recess81R, the second stack-type recess 82R, and the third stack-type recess83R. The first stack-type recess 81R may include a stack of a firstlower recess 81L and a first upper recess 81U, and may further include afirst interconnection recess 811 interconnecting the first lower recess81L and the first upper recess 81U. The second stack-type recess 82R mayinclude a stack of a second lower recess 82L and a second upper recess82U, and may further include a second interconnection recess 821interconnecting the second lower recess 82L and the second upper recess82U. The third stack-type recess 83R may include a stack of a thirdlower recess 83L and a third upper recess 83U, and may further include athird interconnection recess 831 interconnecting the third lower recess83L and the third upper recess 83U. The first to third interconnectionrecesses 811 to 831 may correspond to the first to third air gaps V1 toV3. Therefore, the air gaps V1 to V3 may be embedded in the inside ofthe first stack-type recess 81R, the second stack-type recess 82R, andthe third stack-type recess 83R, respectively.

The first buffer pad 61P may be positioned between the first lowerrecess 81L and the first upper recess 81U. The second buffer pad 62P maybe positioned between the second lower recess 82L and the second upperrecess 82U. The third buffer pad 63P may be positioned between the thirdlower recess 83L and the third upper recess 83U. The first buffer pad61P may be coupled to the first dielectric layer 21, the second bufferpad 62P may be coupled to the second dielectric layer 22, and the thirdbuffer pad 63P may be coupled to the second dielectric layer 23.

For an embodiment, the first stack-type recess 81R, the secondstack-type recess 82R, and the third stack-type recess 83R do notcontact each other due to the dielectric layers 21 to 23, the cappinglayer 71, and the first to third buffer pads 61P to 63P.

As described above, the stack-type recesses 81R, 82R, and 83R may beformed in the contact region CR. The recesses 81, 82, and 83 formed inthe cell array region MC may be single-type recesses. The stackingrecesses 81R, 82R, and 83R may be thicker than the recesses 81, 82, and83 formed in the cell array region MC. The stack-type recesses 81R, 82R,and 83R may be thick recesses, and the recesses 81, 82, and 83 formed inthe cell array region MC may be relatively thin recesses.

Referring to FIG. 5N, a plurality of gate electrodes 91G, 92G, and 93Gmay be formed. The gate electrodes 91G to 93G may be formed by fillingthe recesses 81, 82, and 83, the air gaps V1 to V3, and the stack-typerecesses 81R, 82R, and 83R with a conductive material. This may bereferred to as ‘an inner filling process’. The gate electrodes 91G to93G may include tungsten. According to an embodiment, the gateelectrodes 91G to 93G may be a stack of titanium nitride/tungsten(TiN/W). The gate electrodes 91G to 93G may surround the verticalchannel structure 13.

The gate electrodes 91G to 93G may include a first gate electrode 91G, asecond gate electrode 92G, and a third gate electrode 93G. The firstgate electrode 91G, the second gate electrode 92G, and the third gateelectrode 93G may be vertically stacked with the dielectric layers 21 to23 interposed between them.

The first gate electrode 91G may include a first base portion 91B and afirst pad portion 91P. The second gate electrode 92G may include asecond base portion 92B and a second pad portion 92P. The third gateelectrode 93G may include a third base portion 93B and a third padportion 93P. The first pad portion 91P may extend from the end of thefirst base portion 91B. The second pad portion 92P may extend from theend of the second base portion 92B. The third pad portion 93P may extendfrom the end of the third base portion 93B. The first pad portion 91P,the second pad portion 92P, and the third pad portion 93P may bepositioned in the contact region CR. The first base portion 91B, thesecond base portion 92B, and the third base portion 93B may have thesame thickness. The first pad portion 91P, the second pad portion 92P,and the third pad portion 93P may have the same thickness. The first padportion 91P, the second pad portion 92P, and the third pad portion 93Pmay be thicker than the first base portion 91B, the second base portion92B, and the third base portion 93B, respectively. Each of the firstbase portion 91B, the second base portion 92B, and the third baseportion 93B may be referred to as a single-layered gate electrode. Eachof the first pad portion 91P, the second pad portion 92P, and the thirdpad portion 93P may be referred to as a multi-layered pad portion.

Each of the first pad portion 91P, the second pad portion 92P, and thethird pad portion 93P may have a double pad structure. The first padportion 91P may include a first lower pad 91L, a first upper pad 91U,and a first pad interconnection portion 911 interconnecting the firstlower pad 91L and the first upper pad 91U. The second pad portion 92Pmay include a second lower pad 92L, a second upper pad 92U, and a secondpad interconnection portion 921 interconnecting the second lower pad 92Land the second upper pad 92U. The third pad portion 93P may include athird lower pad 93L, a third upper pad 93U, and a third padinterconnection portion 931 interconnecting the third lower pad 93L andthe third upper pad 93U. The first upper pad 91U, the second upper pad92U, and the third upper pad 93U may be spaced apart from each other bythe capping layer 71. The first lower pad 91L and the first upper pad91U may have the same thickness, and may be thicker than the first baseportion 91B. The second lower pad 92L and the second upper pad portion92U may have the same thickness and may be thicker than the second baseportion 92B. The third lower pad 93L and the third upper pad 93U mayhave the same thickness and may be thicker than the third base portion93B. The first lower pad 91L, the second lower pad 92L, and the thirdlower pad 93L may have the same thickness. The first upper pad 91U, thesecond upper pad 92U, and the third upper pad 93U may have the samethickness.

The first pad portion 91P, the second pad portion 92P, and the third padportion 93P may further include a first buffer pad 61P, a second bufferpad 62P, and a third buffer pad 63P, respectively. The first buffer pad61P may be positioned between the first lower pad 91L and the firstupper pad 91U. The second buffer pad 62P may be positioned between thesecond lower pad 92L and the second upper pad 92U. The third buffer pad63P may be positioned between the third lower pad 93L and the thirdupper pad 93U. The first buffer pad 61P, the second buffer pad 62P, andthe third buffer pad 63P may be in contact with the first dielectriclayer 21, the second dielectric layer 22, and the third dielectric layer23, respectively.

The first pad portion 91P, the second pad portion 92P, and the third padportion 93P may have a stepwise structure. The upper surface of thefirst pad portion 91P may be positioned at a level lower than the uppersurface of the second pad portion 92P, and the upper surface of thesecond pad portion 92P may be positioned at a level lower than the uppersurface of the third pad portion 93P.

Referring to FIG. 5O, a plurality of contact holes 94, 95, and 96 may beformed. The contact holes 94, 95, and 96 may be formed by using acontact mask (not shown) and etching the planarization layer 72 and thecapping layer 71. For example, the contact holes 94, 95, and 96 mayinclude a first contact hole 94, a second contact hole 95, and a thirdcontact hole 96. The first contact hole 94 may expose the first upperpad 91U, the second contact hole 95 may expose the second upper pad 92U,and the third contact hole 96 may expose the third upper pad 93U. Thedepths of the first contact hole 94, the second contact hole 95, and thethird contact hole 96 may be different from each other.

When the first contact hole 94, the second contact hole 95, and thethird contact hole 96 are formed, the first buffer pad 61P, the secondbuffer pad 62P, and the third buffer pad 63P may serve as a supportinglayer. The first buffer pad 61P, the second buffer pad 62P, and thethird buffer pad 63P may increase the structural stability of the firstpad portion 91P, the second pad portion 92P, and the third pad portion93P. Also, when the first contact hole 94, the second contact hole 95,and the third contact hole 96 are formed, the first buffer pad 61P, thesecond buffer pad 62P, and the third buffer pad 63P may serve as etchstop layers. The first buffer pad 61P, the second buffer pad 62P, andthe third buffer pad 63P may suppress the punch phenomenon of the firstpad portion 91P, the second pad portion 92P, and the third pad portion93P.

According to some embodiments, when the contact holes 94, 95, and 96 areformed, punching may be stopped over the lower pads 91L, 92L, and 93Leven when the upper pads 91U, 92U, and 93U are punched. Thus, bridgesbetween the vertically stacked gate electrodes 91G, 92G, and 93G may beprevented.

Referring to FIG. 5P, a plurality of contact plugs 94C, 95C, and 96C maybe formed. The contact plugs 94C, 95C, and 96C may be formed by fillingthe contact holes 94, 95, and 96 with a conductive material such as ametal layer and then performing a planarization process. The contactplugs 94C, 95C, and 96C may include a first contact plug 94C, a secondcontact plug 95C, and a third contact plug 96C. The first contact plug94C may fill the first contact hole 94, the second contact plug 95C mayfill the second contact hole 95, and the third contact plug 96C may fillthe third contact hole 96. The first contact plug 94C may be in contactwith the first upper pad 91U, the second contact plug 95C may be incontact with the second upper pad 92U, and the third contact plug 96Cmay be in contact with the third upper pad 93U. The first contact plug94C, the second contact plug 95C, and the third contact plug 96C mayhave different heights.

Because the pad portions 91P, 92P, and 93P are formed in the double padstructure, the contact plugs 94C, 95C, and 96C are prevented from beingpunched. As a result, short-circuit failure of the gate electrodes 91G,92G, and 93G is avoided.

Also, because the gate electrodes 91G, 92G, and 93G are thinner in thecell array region MC, more memory cells may be stacked. This may improvethe degree of integration for memory devices.

FIGS. 6A and 6B show cross-sectional views illustrating a pad portionaccording to two embodiments. FIGS. 6A and 6B provide comparativeexamples of the first gate electrode 91G and the first pad portion 91Pof FIG. 5P.

Referring to FIG. 6A, the first gate electrode 91G1 of a firstcomparative example may include a base portion 91B and a first padportion 91P1. The base portion 91B and the first pad portion 91P1 of thefirst gate electrode 91G1 may have the same thickness D1. The first padportion 91P1 may correspond to the first pad portion 91P of FIG. 5P, butthe first pad portion 91P1 may be thinner than the first pad portion 91Pof FIG. 5P.

Because the first pad portion 91P1 has a single pad structure and a thinthickness, it is difficult to suppress the punch phenomenon 97 of thefirst contact plug 94C. The first contact plug 94C may be coupled to thesubstrate 11, as shown, penetrating through the dielectric layer 21 dueto the punch phenomenon 97.

In the first comparative example, the thickness of the first gateelectrode 91G1 may be increased to prevent the punch phenomenon of thefirst pad portion 91P1. However, when the thickness of the first gateelectrode 91G1 is increased, there is a limit in stacking memory cells.In contrast, because the pad portion in accordance with the embodimentsof the present teachings may be formed in a double pad structure or atriple pad structure, the gate electrode may be formed thin. As aresult, the degree of integration of memory cells may be improved.

Referring to FIG. 6B, the first gate electrode 91G2 of a secondcomparative example may include a base portion 91B and a first padportion 91P2. The first pad portion 91P2 may be thicker than the baseportion 91B (D2>D1). The first pad portion 91P2 may correspond to thefirst lower pad 91L of the first pad portion 91P of FIG. 5P.

Because the first pad portion 91P2 has a single pad structure, it isdifficult to suppress the punch phenomenon 97 of the first contact plug94C. The first contact plug 94C may be coupled to the substrate 11through the dielectric layer 21 based on the punch phenomenon 97.

Whereas the first comparative example and the second comparative exampleboth show a single pad structure resulting in the punch phenomenon 97,the first pad portion 91P of FIG. 5P has a double pad structureincluding the first lower pad 91L and the first upper pad 91U.Therefore, the double pad structure may be advantageous in suppressingthe punch phenomenon.

According to the present teachings, when a contact plug is formed, it ispossible to protect a pad portion from being punched by forming the padportion in a multi-layered pad structure at the end of a gate electrode.Therefore, it is possible to prevent a bridge defect that may occur asgate electrodes are penetrated. As a result, semiconductor devices withimproved reliability may be provided.

While the present teachings have been described using a limited numberof possible embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made to presentedembodiments without departing from the spirit and scope of the presentteachings as set forth in the following claims.

What is claimed is:
 1. A method for fabricating a vertical memorydevice, the method comprising: forming a plurality of dielectric layersand a plurality of sacrificial layers interleaved over a substrate in acell array region and a contact region; forming a stepwise structurewhere the sacrificial layers are exposed by patterning the dielectriclayers and the sacrificial layers in the contact region; forming aplurality of multi-layer sacrificial structures including a sacrificialmaterial over the exposed sacrificial layers of the stepwise structure;forming a capping layer over the sacrificial structures, wherein thecapping layer includes a plurality of air gaps respectivelyinterconnecting the multiple layers of multi-layer sacrificialstructures of the plurality of multi-layer sacrificial structures;replacing the sacrificial layers in the cell array region with aplurality of gate electrodes; replacing the sacrificial layers, the airgaps, and the multi-layer sacrificial structures in the contact regionwith a plurality of multi-layered pad portions; and forming a pluralityof contact plugs, wherein contact plugs of the plurality of contactplugs are respectively coupled to multi-layered pad portions of themulti-layered pad portions.
 2. The method of claim 1, wherein themulti-layered pad portions are thicker than the gate electrodes.
 3. Themethod of claim 1, wherein replacing the sacrificial layers, the airgaps, and the multi-layer sacrificial structures in the contact regionwith a plurality of multi-layered pad portions comprises: forming lowerpads and upper pads which are spaced vertically apart from the lowerpads through replacement of the sacrificial layers and the sacrificialmaterial; and forming pad interconnection portions by filling the airgaps to interconnect the lower pads and the upper pads.
 4. The method ofclaim 1, wherein replacing the sacrificial layers in the cell arrayregion with a plurality of gate electrodes and replacing the sacrificiallayers, the air gaps, and the multi-layer sacrificial structures in thecontact region with a plurality of multi-layered pad portions comprises:forming a slit by etching the capping layer, the plurality ofsacrificial layers, and the plurality of dielectric layers; forming aplurality of horizontal recesses by removing the plurality ofsacrificial layers in the cell array region through the slit; forming aplurality of stack-type recesses with the air gaps embedded therein byremoving the plurality of sacrificial layers and the sacrificialmaterial in the contact region through the plurality of horizontalrecesses; and filling the plurality of horizontal recesses and theplurality of stack-type recesses with a conductive material to form theplurality of gate electrodes and the plurality of multi-layered padportions.
 5. The method of claim 1, wherein forming the plurality ofmulti-layer sacrificial structures including a sacrificial material overthe exposed sacrificial layers of the stepwise structure comprisesdepositing a sacrificial pad, then depositing a buffer pad, and thendepositing an additional sacrificial pad over a sacrificial layer of theplurality of sacrificial layers in the contact region.
 6. The method ofclaim 5, wherein the sacrificial layer, the sacrificial pad, the bufferpad, and the additional sacrificial pad are formed of the same material.7. The method of claim 5, wherein the buffer pad and the plurality ofdielectric layers comprise silicon oxide, and wherein the plurality ofsacrificial layers, the sacrificial pad, the buffer pad, and theadditional sacrificial pad comprise silicon nitride.
 8. The method ofclaim 1, wherein forming a plurality of multi-layer sacrificialstructures including a sacrificial material over the exposed sacrificiallayers of the stepwise structure comprises: forming a sacrificial padlayer over a sacrificial layer; etching the sacrificial pad layer toform a sacrificial pad; sequentially depositing a buffer layer and anadditional sacrificial pad layer over the sacrificial pad; forming asacrificial buffer pad over the additional sacrificial pad layer;etching the additional sacrificial pad layer with the sacrificial bufferpad used as an etch mask to form the additional sacrificial pad; andetching the buffer layer to form the buffer pad, wherein the additionalsacrificial pad and the sacrificial pad form a multi-layer sacrificialstructure of the plurality of the multi-layer sacrificial structures. 9.The method of claim 1, further comprising forming, after forming thecapping layer, a vertical channel structure that penetrates through thecapping layer, the plurality of dielectric layers, and the plurality ofsacrificial layers in the cell array region.